In analog oscilloscopes employing cathode ray tubes (CRTs) for display, variable brightness of the screen display communicates useful information about the activity of the signal being observed. As an analog oscilloscope generates vertical excursions during a horizontal sweep interval to provide a real-time picture of the signal activity at the probe tip, it inherently tends to vary the brightness of the display as an inverse function of the slope of the line it produces. This occurs because the cathode electron gun of the CRT generates a constant supply of electrons that depends on the setting of a “brightness” control, and the length of the trajectory covered in a unit of time is minimally determined by the x-axis distance associated with any particular sweep speed, but is increased by any and all y-axis excursions. And a y-axis excursion can be a large multiple of the corresponding x-axis distance, so the constant available electron beam energy appears to be reduced by a large factor as it is spread over this much longer distance. Thus, analog oscilloscopes inherently vary the brightness of the line they draw as an inverse function of the slope of that line.
Another even more highly desired feature of an analog oscilloscope or a digital oscilloscope with a high waveform throughput, is the ability to detect an intermittent signal anomaly that occurs in an otherwise repetitive signal. Older digital oscilloscopes, with low “live time” make observing intermittent signal activity improbable, at least in the absence of special trigger modes designed to detect certain classes of intermittent signal activity. Analog oscilloscopes will show a faint trace indicating the presence of this intermittent anomalous signal behavior. Of course, if the signal becomes too intermittent, the trace will be so faint in brightness that it may be missed entirely by the oscilloscope operator.
In a simple digital oscilloscope having only a single bit (on/off) of intensity information per pixel, operated with its persistence decay feature turned off (i.e., infinite persistence), will display rare or unusual waveforms with the same intensity as highly repetitive ones, i.e., “on”. In contrast, digital oscilloscopes with multi-bit raster memories can provide variable intensity (or variable color) displays to allow a visual distinction to be made between rare and repetitive waveforms. However, unless the persistence feature is turned off, these oscilloscopes may not illuminate truly rare events with enough intensity for a long enough period of time to allow the operator to notice, much less analyze, the intermittent activity.
Current oscilloscope products from assignee corporation, Tektronix, provide a means whereby the operator can distinguish between the most recent individual waveform acquisitions and older waveforms acquired previously. The Tektronix TDS3000 oscilloscope defines and detects unusual waveforms it acquires in a way that allows the definition of an unusual waveform to be controlled by the operator while the detection of the waveforms is then automatically performed by the oscilloscope. U.S. Pat. No. 6,163,758 (Sullivan, et al.), entitled Detection of Unusual Waveforms, assigned to the same assignee as the subject invention, discloses the manner in which such unusual waveforms are detected. Specifically, it detects unusual waveforms by counting the number of new pixels that are drawn on its screen display, and generating an alert signal if the number of new pixels exceeds a threshold value. That is, the number of “new” pixels that unusual or anomalous waveforms affect automatically distinguish them from normal, highly repetitive, waveforms. “New” pixels can be defined to mean those pixels that have never been previously touched by any waveform since the beginning of the present acquisition series, or those which have not been affected for some interval of time as measured by the decay of values stored in a raster memory. User input can affect the definition of unusual waveforms by affecting the definition of “new” pixels (i.e., threshold value) and by affecting the decay rate applied to the values stored in the raster memory.
While the system of U.S. Pat. No. 6,163,758 performs well, what is needed is a system that determines the threshold and updates it dynamically, and in a post-detection mechanism also discards unusual waveforms that are due to noise rather than a significant anomaly.